1. Field of the Invention
The field of the invention generally relates to fabrication of silicon wafers. In particular, the field of the invention generally relates to a method and apparatus for forming a silicon wafer of extremely high purity by melting silicon in a crucible comprising cooled aluminum or other suitable material such that a non-wetted interface is created wherein the silicon does not wet nor interact with the surface of the crucible.
2. Background of Related Art
Silicon is the basic raw material of the semiconductor and solar cell industries. Second only to oxygen, silicon is the most common material on earth. However, it does not occur in the free state, only in myriad of mineral combinations. Silicon must thus be refined and purified for industrial use.
Silicon is used for other industries, also, such as metals, glasses, and ceramics. For these industries, a quite acceptable purity level is 99 percent (i.e., 1% impurity). For semiconductors, however, the purity must be far greater; on the order of 99.9999999% or one part per billion impurity. Even part per trillion impurity levels are of interest. The reason for emphasis upon such intense purity is that electronic properties of a material work at the atomic level. Even at one part per trillion impurity, a single cubic centimeter still contains about ten billion impurity atoms, which interfere with performance.
Solar cells are also semiconductor devices. They, too, benefit from high purity, but can tolerate somewhat lower purity. Thus, an industrial benchmark at this time for semiconductors is one part per billion (ppb), solar one part per million (ppm), metallurgical one part per hundred (percent).
The metallurgical grade silicon is thus likely to be the feedstock for purification for semiconductor and solar applications. Decades of intense development and experience have achieved the required minimal purity.
At this stage the silicon is in the form of spherical beads about one millimeter in diameter. Since both solar and semiconductor applications require a silicon sheet, the next step is consolidation of the beads into sheet form. The heat required to do this however, is highly likely to degrade the purity that has taken so much time and effort to achieve. Consolidation requires molten conditions. Molten silicon must be contained in a containment structure. At 1410° centigrade (silicon melting point), there really is no known material suitable to hold the melt. Molten silicon either dissolves or chemically attacks the container, leading to contamination as well as damage to expensive apparatus. Every effort is made to reduce the problem, but clearly this stage of silicon preparation is undesirably expensive and complicated.
Unfortunately, exceedingly high purity isn't enough, the silicon also must be crystalline. This means the individual silicon atoms must be arranged in a regular geometric pattern or lattice structure.
In the most expensive and exotic approach (Czochralski crystal pullers), a large crucible comprising high purity silica (melting point 1710° C.), silicon dioxide, sometimes called quartz, or silicon carbide (melting point 2600° C.) is heated under a protective atmosphere or vacuum to the melting point of silicon (1410° C.).
At this temperature, the silicon is very active and very inclined to chemically react with or dissolve its surroundings. In the melt region, portions of the silica and silicon carbide in contact with the molten silicon are dissolved, and reaction with any residual gasses or other impurities in the melt area will occur. This contaminates the silicon and limits the lifetime of the expensive crucibles.
The Czochralski apparatus is provided with means for bringing a small piece of silicon crystal in contact with the melt. If the temperature is carefully controlled, the crystal, instead of melting, prompts the solidification of melt material onto the crystal. If the rate is also carefully controlled, the solidification process copies the lattice geometry of the seed crystal and a large single crystal of silicon begins forming. As the solidified material is slowly pulled away, the melt is transformed into a large piece of single crystal silicon (sometimes called a boule).
The physical meaning of “melting” is that heat energy added to individual atoms (or molecules) eventually adds mobility to the point where the atoms are no longer bound by cohesive (solidification) forces. They are now liberated to diffuse freely. If careful cooling is now applied, the atoms organize themselves into the geometric lattice of the crystal structure. Simultaneously, impurity atoms, which have different sizes and cohesive forces associated with them, tend to be excluded from the organized crystallization. The Czochralski process thus has crystallization starting and progressing from a single point (so multiple crystals don't form) while systematically excluding contaminants.
To provide the silicon sheets, the boule of single crystal silicon must be sliced into sheets or wafers. In the Czochralski process, the slicing or sawing process results in the loss of 40 or 50 percent or more (in saw kerf) of this exotic, expensive material. Cleaning of the saw-cut surface by grinding, chemical etching and polishing also may be required. Clearly, such wafers are a remarkable—but expensive—achievement.
When the seed crystal is contacted to the melt in the Czochralski reactor, the melt region is quite small; some few millimeters in dimension. The pulling process, properly controlled, allows the solidification process to taper outward towards a larger diameter. This diameter typically shows prismatic features indicative of the atomic scale crystal organization and determines the size of wafers that can be produced. Years of development effort have moved towards larger diameter boules to the present state of the art of about 12 inches. The length of the boule is limited by the weight that can be held by the tail near the original seed and the volume of the Czochralski crucible or reactor. The outer part of the boule is ground off to a consistent diameter and also to remove a region that is higher in impurities. The tapered tails are also lost for production. These are losses which add to the saw kerf loss.
For solar cells, which may require large, i.e., greater than 12 inches, area, or a large length to width aspect ratio (for improved interconnects), such sizes of silicon sheet are simply unavailable. Work has been done on the pulling of silicon ribbon, rather than a boule, to reduce expense and kerf losses, but the ribbon technique has been plagued by quality and productivity problems.
In another conventional method known as edge-defined film fed growth (EFG), a silicon film or layer is formed without the need for pulling a crystal as in the CZ method. In some cases, the silicon sheet is formed from a nozzle structure comprising refractory materials such as molybdenum, tungsten or the like. However, in such instances the nozzle becomes a wetted structure and undesirable molybdenum silicon compounds are formed in the silicon.
If silicon feed stock of high purity beads can be melted in a suitable container, varieties of casting and molds can be used for solidification. Sawing is still required, material is multi-crystal rather than single crystal and purity is compromised. Such material is not suitable for semiconductors, but can be used for solar cells at the expense of efficiency and long-term reliability.
Another chronic issue is the aggressive nature of molten silicon as it reacts chemically with the crucible, leading to contamination and equipment damage. Therefore, what is needed is a new method for making a silicon sheet of high purity in a cost-effective manner, while eliminating sources of contamination.
Although great progress has been made in the availability of suitable silicon, many difficulties remain and much improvement is needed to provide silicon wafers of high purity and cost effectiveness.
What is needed is a new method for high-temperature melting of silicon, while preventing contamination by oxygen and other impurities, such as surface interaction with a containment vessel during the melting process. Such a simplified process for making a silicon wafer would minimize the cost of the ultimate product, thus expanding the opportunity for widespread practical application of silicon wafers in areas such as solar energy production.